1. Chapter 15 Populations Coach Fults Biology

3. What is a Population? A population consists of all the individuals of a species that live together in one place at the same time Populations tend to grow b/c individuals have multiple offspring over their lifetime But limited resources in the environment can reduce the population Demography- the statistical study of all the populations Demographers try to predict how populations will change over time

4. 3 Key Features of Populations 1. Population size- # of individuals in a population –Small populations tend to be the first that go extinct –Random events or natural disasters can hurt the populations much worse than larger populations –They also have a higher rate of interbreeding, which most individuals have the same genetics, which reduce their fitness –More individuals will be homozygous for recessive traits

5. 3 Key Features of Populations 2. Population density- the number of individuals that live in a given area –If the individuals of a population are few and are spaced widely apart, they may seldom encounter each other, making reproduction rare

7. 3 Key Features of Populations 3. Dispersion- the way the individuals are arranged in their space –If the individuals are randomly spaced, the location of each individual is self-determined or determined by chance –If the individuals are evenly spaced, they are located at regular intervals –In a clumped distribution, individuals are bunched together in clusters –Each of these patterns reflect the interactions between the population and its environment

8. Modeling Population Growth When demographers try to predict how a population will grow, they make a model of the population Population model- is a hypothetical population that attempts to exhibit key characteristics of a real population By making changes in the model they can see how the outcomes differ, and to what could happen to the population To learn how demographers study a population, consider a simple model of growth in 3 stages of complexity

10. 1 st Stage Growth Rate A population grows when more individuals are born than die in a given period So a simple population model describes the rate of population growth as the difference between birthrate and death rate. They are usually expressed as the number of births and deaths per thousand people per year

12. 2 nd Stage Growth Rate and Population Size When population size is plotted against time on a graph, the population growth curve resembles a J- shaped curve and is called an exponential growth curve Exponential growth curve- is a curve in which the rate of population growth stays the same, as a result the population increases steadily To calculate the # of individuals that will be added to the population as it grows, multiply the size of the current population (N) by the rate of growth (r)

14. 2 nd Stage Growth Rate and Population Size However, populations do no usually grow unchecked Predators, disease, limited resources Eventually, growth slows, & the population will stabilize Carrying capacity- the population size that an environment can sustain

16. 3 rd Stage Resources and Population size As a population grows, limited resources can be depleted, when this happens the population slows The population model can be adjusted to account for the effect of limited resources, such as food and water These resources are called density-dependent factors because the rate at which they become depleted depends upon the population density of the population that uses them

17. 3 rd Stage Resources and Population size The population model that takes into account the declining resources available to populations is called the logistic model of population growth, after the mathematical form of an equation The logistic model- (S-curve)is a population model in which exponential growth is limited by a density- dependent factor The everyday meaning of logistics refers to the ability to obtain, maintain, & transport materials

19. 3 rd Stage Resources and Population size In other words, logistics is about solving day-to-day problems of living Unlike simple models, logistic models assume that birth/death rates vary with population size When a population is below carrying capacity, the growth rate is rapid, however as they closer to the carrying capacity the death rates grow and the birth rates decline; as a result the growth rate slows. Growth rate will stop when # of births=# of deaths

20. 3 rd Stage Resources and Population size The logistic model of population growth, though simple, provides excellent estimates of how populations grow in nature Competition for food, shelter, mates, & resources tend to increase as a population approaches its carrying capacity P.323 figure 5 summarizes what I have said

21. Growth Patterns in Real Populations Many species of plants and insects reproduce rapidly. Their growth is usually limited not by density-dependent factors but by environmental conditions, also known as density-independent factors Weather and climate Most species have a strategy (pattern of living) somewhere in between the 2 models; other species change from one to the other as the environment changes

22. Rapidly Growing Populations Many species, including bacteria, some plants, and many insects are found in rapidly changing environments Such species, called r-strategists, grow exponentially when the environmental conditions allow them to reproduce This strategy results in temporary large populations. When environmental conditions worsen, the population drops quickly They have short life-span, reproduce fast, small offspring with little or no parental care ROACHES

24. Slowly Growing Populations K-strategists- grow slow, small population size, WHALES; their population size is near the carrying capacity for the environment Long life span, slow maturing process, reproduce late in life, few offspring, and provide care to their offspring in a stable environment Tigers & gorillas are also examples

26. The Change of Population Allele Frequencies In the 100 years, after Darwin died, the science of genetics has allowed biologists to construct models of how natural selection alters the proportions of alleles within populations But b4, you can understand how populations change in response to evolutionary forces, you need to learn how populations behave in the absence of these forces

27. Allele Frequencies When Gregory Mendel’s work was rediscovered in 1900, biologist began to study how frequencies of alleles change in a population. Specifically, they wondered if dominant alleles, which are usually more common than recessive alleles, would spontaneously replace recessive alleles within populations 1908, the English mathematician G.H. Hardy and the German physician Wilhelm Weinberg independently demonstrated that dominant alleles do not automatically replace recessive alleles

28. Allele Frequencies Using algebra and simple application of the theories of probability, they showed that the frequency of alleles in a population does not change Moreover; the ratio of heterozygous individuals to homozygous individuals do not change from generation to generation unless the population is acted on by process that favor particular alleles If a dominant allele is lethal, for example, it will not become more common just because it is dominant

29. Allele Frequencies Their discovery, called the Hardy-Weinberg principle, states that the frequencies of alleles in a populations do not change unless evolutionary forces act on a population

30. The Hardy-Weinberg Principle The principle holds true for any population as long as the population is large enough that its members are not likely to mate with relatives and as long as evolutionary forces are not acting There are 5 principle evolutionary forces: mutation, gene flow, nonrandom mating, genetic drift, and natural selection These forces can cause the ratios of genotypes in a population to differ significantly from those predicted by Hardy-Weinberg principle

31. The Hardy-Weinberg Principle p 2 +2pq +q 2 =1 Frequency of individuals that are homozygous for allele A; plus: frequency of heterozygous individuals with alleles A and a; plus; frequency of individuals that are homozygous for allele a

33. Mutation Although mutation from one allele to another can eventually change allele frequencies, mutations rates in nature are very slow Most genes mutate about 1 to 10 times per 100,000 cell divisions, so mutations do not change allele frequencies, except over a long period of time Not all mutations result in a change in phenotype Recall that more than one codon codes for a certain protein, and other changes in an amino acid that do occur may not affect how the protein works.

35. Mutation It is the source of variation and thus makes evolution possible

36. Gene Flow The movement of individuals from one population to another can cause genetic change. The movement of individuals to or from a population, called migration, creates gene flow, the movement of alleles into or out of a population This occurs because new immigrants bring in new alleles and emigrants take away alleles

38. Nonrandom Mating Sometimes individuals prefer to mate with others that live nearby or are of their own phenotype, a situation called nonrandom mating Mating with relatives(inbreeding) is a type of nonrandom mating that causes a lower frequency of heterozygotes than would be predicted by the Hardy-Weinberg principle Inbreeding does not change the frequencies of alleles, but it does increase the population of homozygous individuals

39. Nonrandom Mating Nonrandom mating also results when organisms choose their mates based on certain traits. In animals, females often select males based on their size, color, ability to gather food, or other characteristics

41. Genetic Drift In small populations the frequency of an allele can be greatly changed by a chance event For example, a fire can reduce a large population to a few survivors. When an allele is found in a few individuals, the loss of even one individual from the population can have major effects on the allele’s frequency. Because this sort of change in allele frequency appears to occur randomly, as if the frequency were drifting, it is called genetic drift

42. Genetic Drift Small populations that are isolated from one another can differ greatly as a result of genetic drift Cheetahs, whose evolution has been seriously affected by genetic drift; undergone drastic population declines over the past 5,000 years As a result cheetahs alive today are descendants of only a few individuals, and each cheetah is almost genetically uniform with other members of the population

43. Genetic Drift One consequence of this genetic uniformity is reduced disease resistance- cheetah cubs are more likely to die from disease than are tiger and lion cubs This reduction in genetic diversity of cheetahs may hasten their extinction

45. Natural Selection Natural selection causes deviations from the Hardy- Weinberg proportions by directly changing frequencies of alleles The frequency of an allele will increase or decrease, depending on the allele’s effects on survival and reproduction. For example, the allele for sickle cell anemia is slowly declining in frequency in the U.S. because individuals who are homozygous for the allele rarely have children. Heterozygotes are resistant to malaria, a significant health problem in the world

46. Natural Selection As a result, homozygotes are selected against in the United States, and the frequency of the sickle cell anemia alleles decreases Natural selection is one of the most powerful agents for genetic change

48. Action of Natural Selection Natural selection constantly changes populations through actions on individuals within the populations However, natural selection does not act directly on genes; it enables individuals with those favorable genes to reproduce and pass those traits on to their offspring This means that natural selection acts on phenotypes not genotypes

49. How Selection Acts Think carefully how N.S. might operate on a mutant allele. Only characteristics that are expressed can be targets of N.S. Therefore, selection cannot operate against rare recessive alleles, even though they are unfavorable Only when the allele becomes common enough that heterozygous individuals come together to produce a homozygous offspring does natural selection have the opportunity to act

50. How Selection Acts In your book, it talks about a family that had one child out of 5 that had hemophilia (bleeder) which is a recessive condition Without good medical attention this can cause uncontrollable bleeding or even death This kind of selection would remove a homozygous person from the gene pool, however; therefore it does not act on the heterozygotes so the gene is not eliminated from the population

51. Why Genes Persist Genetic conditions are not eliminated by natural selection because very few of the individuals bearing the alleles express the recessive phenotypes

52. Natural Selection and the Distribution of Traits Natural selection shapes populations affected by phenotypes that are controlled by 1 or by a large number of genes A trait that is influenced by several genes is called polygenic traits Human height,skin, eye color = are influenced by dozens of genes Natural selection can change the allele frequencies of many different genes governing a single trait, influencing the strongest traits for their phenotype

53. Natural Selection and the Distribution of Traits Like following 1 duck in a flock, it is difficult to keep track of a particular gene. Biologists measure changes in a polygenic trait by measuring each individual in the population These measurements are then used to calculate the average value of the trait for the population as a whole B/c genes can have many alleles, polygenic traits tend to exhibit a range of phenotypes clustered around an average value. If you were to plot the height of everyone in your class on a graph, the values would probably for a hill- shaped curve called a normal distribution

55. Directional Selection When selection eliminates one extreme from a range of phenotypes, the alleles promoting this extreme become less common in the population In 1 experiment, when fruit flies raised in the dark were exposed to a light, some flew toward the light and some did not. Only those that flew to the light were allowed to mate. After 20 generations, the average tendency toward the light increased In directional selection, the frequency of a particular traits moves in 1 direction in a range Like pesticide resistance; evolution in single-gene traits

56. Stabilizing Selection When selection reduces extremes in a range of phenotypes, the frequencies of the intermediate phenotypes increase As a result, the population contains fewer individuals that have alleles promoting extreme types In stabilizing selection- the distribution becomes narrower, tending to “stabilize” the average by increasing the proportion of similar individuals This is very common in nature